Drug molecules are covalently bound to macromolecular carriers in order to enhance pharmaceutical properties, such as half-life, stability, solubility, tolerability, and safety. In one method, drug moieties are coupled to macromolecules through a permanent linker, but this approach is limited by at least two factor: 1) the linker must be attached to the drug moiety at a site that does not hinder biological activity, and 2) permanent conjugates generally cannot cross the cell membrane, so the approach may only be feasible for extracellular drug targets.
In a second approach, covalently bound drug-macromolecule conjugates employ controlled release of drugs or growth factors useful in medicine. For example, compositions and methods have been described for controlled release of drugs covalently coupled to polyethylene glycol (PEG). This approach does not require an extracellular drug target, and the linkage site is preferably one that does hinder the drug's activity so that it is somewhat masked until it is released from the carrier. Typically, for alcohol- or phenol-containing drugs, the drug is attached to the carrier by an ester or carbonate linkage that can be hydrolyzed, usually by a serum esterase. Examples are PEG-camptothecin, PEG-SN38, PEG-irinotecan and PEG-docetaxel. Adaptations have been made to accommodate amine-containing drugs whereby a PEG moiety is connected by a cleavable ester to a self-immolating carbamate. This technology has been applied to peptides and proteins as well as to daunorubicin, amphotericin, Ara-C and other small molecules. However, drug release rates in these cases is unpredictable and difficult to adjust, because esterase activity varies between species and individuals, and certain compartments are esterase-deficient (e.g., topical, intra-ocular, interstitial areas).
Herein, a drug conjugate system is described which allows for drug release through a rate-controlled, beta-elimination mechanism.
The Weizmann Institute developed a system in which a protein or polymer carrier is attached to linkers such as fluorenylmethoxycarbonyl (Fmoc) or its 2-sulfo derivative (Fms). These are described in U.S. Pat. No. 7,585,837. These linkers release drugs via a non-enzymatic beta-elimination mechanism; however, tunable control over the release rate remains a problem with this system.
PCT publication WO2009/158668 describes releasable drug conjugates to macromolecules wherein the rate of beta-elimination is controlled by a trigger independent of the macromolecule itself. This solves a problem left unsolved in the prior art. The '668 PCT publication provides for linkers that are most directly applicable to basic amine-containing drugs, as the linkers attach to the drugs via carbamate or thiocarbamate linkages. There is thus an unmet need for new cleavable linkers that allow for conjugation of alcohol-, phenol-, thiol-, thiophenol-, and certain nitrogen-containing drugs to macromolecular carriers, and that allow for subsequent release of these drugs at controlled rates under physiological conditions. Notably, over 50% of FDA-approved drugs under a molecular weight of 1200 have an primary or secondary aliphatic hydroxyl, phenol, or thiol group, and approximately 8% contain a sulfonamide, pyrrole, or indole nitrogen (see DrugBank). The present invention meets this need by providing drug-macromolecule conjugates linked via N-oxymethyl, N-(heterocyclic amino)methyl, and N-thiomethyl carbamates that are cleavable by beta-elimination under controlled rates to provide the free drugs. Furthermore, the linkers described herein release the free drug molecules without relying on the activity or presence of physiological enzymes.
While simple N-alkoxymethyl, N-acyloxymethyl, and N-thiomethyl carbamates have been previously reported as prodrugs [see, for example, N-acyloxymethyl carbamates described by Majumdar & Sloan, Bioorg. Med. Chem. Lett. (2007) 17: 1447-1450] or as modifiers for textiles [see, for example, U.S. Pat. Nos. 4,539,008; 4,200,564; and 3,758,554], such compounds do not have the ability to release the free drug molecules by rate-controlled beta-elimination.